Glaucoma is a disease which results in elevated intraocular pressure (IOP) and causes damage to the optic nerve. and loss of vision. The usual test for glaucoma is to measure the IOP of a patient's eye using a tonometer, and several forms of tonometers are currently available. One form of a tonometer is disclosed in Zeimer, U.S. Pat. No. 5,546,941 in which a tubular probe having a transverse membrane at the forward end thereof is applied to the eye of a patient, and pressure within the probe is increased until applanation is detected. To ensure accuracy, three or four tests are conducted on each eye during a test sequence. Also, IOP is not constant through the course of a day and, therefore, it is desirable to take several series of tests spaced over a 24 hour period to accurately determine the average IOP of a patient's eye.
Since the membrane at the forward end of the probe contacts the eye during the series of tests, it is desirable to replace the probe, or at least the membrane, before conducting each series of tests. It would, therefore, be desirable to provide a probe which can be used to determine applanation of an eye and having a membrane which is readily replaceable.
The membranes which contact the surface of the eye, however, must have planar surfaces such that when applanation occurs, light is reflected from the inner surface of the membrane down the length of the probe. If the inner surface of the transverse portion is distorted at applanation, light will be reflected into the walls of the probe, and the light detector of the tonometer will not detect a definable peak indicative of applanation. It has been found that a membrane consisting of a tubular portion fitted around a complementary tubular end of a probe, and a transverse end adjacent the tubular end of the probe, will have distortion in the transverse portion, such that the inner surface is not planar at applanation. Also, it has been found that if such a membrane is not carefully fitted to the tubular end of the probe, the transverse portion of the membrane may not be perfectly perpendicular to the axis of the probe such that even if there were no distortion from uneven stretching of the membrane, light would not be reflected down the probe at applanation.
It would, therefore, be desirable to provide a probe with a replaceable membrane which is easily installed and which will not be subject to misalignment on the probe or distortion of the transverse portion thereof.